28 Proceedings of Learned Societies. 



migratory flights by the parents, but must be guided by some un- 

 known principle : if it be admitted in the case of Swifts, Martins, 

 and other birds associating together in flocks, that the young may 

 be directed by the motions of their fellows, yet this cannot be the 

 case with the Nightingales ; nor with the Cuckoos, who, though 

 reared in the nests of many different birds, are regular migrators. 

 The parent Cuckoo has left the country before its young are rear- 

 ed, always departing early in July. 



Dr. Jenner next gave some particulars relative to the enlargement 

 of the testes and ovaria in birds, supplementary to those which have 

 been pointed out by Mr. John Hunter. In those birds who pair but 

 for a short time the testes are small, while in those with whom the 

 connubial compact is of long continuance, they are large. In the 

 Cuckoo, a polygami?t, and who continues with the female but for 

 a very short time, the testes are of the size of a vetch only ; but 

 in the Wren, whose attachment to his mate extends from spring to 

 autumn, they are equal to a pea in magnitude ; thus much larger 

 in the latter than in the former, in proportion to the size of the 

 bird. A continued supply of generative power is required in birds 

 who pair for a long time, in case the brood should be destroyed — 

 but in those like the Cuckoo this provision is unnecessary. 



The winter birds of passage leave this country for precisely the 

 same reason that impels the spring migrators to come hither ; some 

 of them, as the Wild-duck and the Wood-pigeon, which occa- 

 sionally build here, are irregular in their migration ; the most re- 

 gular are the Red-wing and the Field-fare, of whose building in 

 this country Dr. Jenner never met with an instance. The food of 

 the former, he observed, is not haws, or the fruit of the white 

 thorn, as has been stated, but worms and insects, which they 

 gather from the ground, feeding in flocks ; Dr. J. had seen them 

 dying of famine when haws were abundant. A gentleman saw a 

 flock of Field-fares on the day before the thawing of the great 

 frost of 1794, and they seemed as wild and vigorous as if in sea- 

 son ; he shot one, which Dr. Jenner examined, and found to be 

 in excellent condition, but there was no food in the stomach, and 

 the last which the animal had eaten was digested: now as the 

 ground was covered with snow, and as the long frost had destroy- 



